1. Field of the Invention
This invention relates to a two-cycle internal combustion engine of the fuel injection type, and in particular to a two-cycle internal combustion engine of the fuel injection type which is relatively small in size and suited for use in a portable working machine such as a chain saw or a bush cutter.
2. The Prior Art
Because of the increasing concern in recent years of environmental problems, a reduction not only of the toxic substances in the exhaust gas but also of the engine noise is strongly demanded even in a small two-cycle internal combustion engine. In particular, there are pollution problems inherent to a two-cycle internal combustion engine, i.e. a problem of how to reduce the quantity of HC in the exhaust gas which is brought about due to a phenomenon of blow-by of unburnt air-fuel mixture from the combustion chamber, and a problem of how to prevent the discharge of the unburnt air-fuel mixture which is brought about due to a phenomenon of spitting of fuel toward the air-cleaner. These problems are also desired to be solved.
On the other hand, a lean burn engine or a direct injection engine, wherein fuel is directly injected into a combustion chamber, has been recently proposed as a four-cycle internal combustion engine. In these types of engine, the reduction of toxic substances in the exhaust gas by means of a lean-burning technique is taken into consideration Therefore, it is now studied how to apply this technique to a two-cycle internal combustion engine.
The conventional two-cycle internal combustion engine of relatively small size (total displacement. 60 cc or less) which is now employed in a portable working machine such as a chain saw or a bush cutter, is generally composed of a cylinder portion having a cylindrical inner wall, so as to allow a piston to be slidable fitted therein, a head portion provided with a combustion chamber of the so-called squish dome type, and a plurality of air-cooling fins formed on the outer peripheries of these cylinder and head portion. The cylinder portion is provided with a suction port and an exhaust port which are disposed respectively at the upper and lower portion of the cylinder portion so as to face to each other. A pair of scavenging ports are also formed in the inner wall of the cylinder portion so as to face to each other, the openings of these scavenging ports being located to intersect at a right angle with the suction port and exhaust port. There is also known an internal combustion engine which is aimed at facilitating the monoblock casting of the cylinder portion and the head portion by means of a high pressure die casting, wherein a pair of openings for casting are formed along each scavenging passage communicating with the scavenging port so as to expose each scavenging passage. In this case, a pair of scavenging passage covers, each having a smoothly curved inner surface in conformity with the scavenging passage and prepared separately from the cylinder portion, is attached to the openings-for-casting, respectively, so as to close the openings-for-casting, thereby forming smoothly curved scavenging passages.
On the other hand, a two-cycle internal combustion engine of the direct fuel injection type is also proposed, wherein a fuel injection nozzle is disposed at a cylinder head portion of the combustion chamber so as to permit the injection of fuel to be effected directly to the combustion chamber. (See, for example, U.S. Pat. No. 4,813,391.) In this two cycle internal combustion engine, a fuel pump is actuated taking advantage of the fluctuation in pressure within a crank case. Through such actuation of the fuel pump, fuel is fed to the fuel injection nozzle and then directly injected into the combustion chamber, thereby allowing the fuel to be mixed with suction air that has been sucked via the scavenging port into the combustion chamber, the resultant air-fuel mixture being ignited and burnt.
There is also known another example of a two-cycle internal combustion engine of the direct fuel injection type, wherein a fuel injection nozzle is disposed midway of an air passage (scavenging passage) communicating with the combustion chamber. Fuel from the injection nozzle is injected toward a heat conductive wall constituting a cylinder wall and allowed to impinge thereon so as to be gasified and mixed with air passing through the air passage, the resultant air-fuel mixture being fed to combustion chamber (See U.S. Pat. No. 4,876,999).
However, in the case of the former engine of direct fuel injection type (See U.S. Pat. No. 4,813,391) wherein the fuel injection nozzle is mounted on the cylinder body and fuel is directly injected from the fuel injection nozzle into the combustion chamber, a hole for mounting the injection nozzle and communicating with the interior of the cylinder body is required to be formed in the cylinder body. Namely, a troublesome working of the cylinder body is required to be performed. At the same time, since the injection nozzle is mounted on the combustion chamber portion of the cylinder body, the heat from the combustion chamber is conducted through the cylinder body to the injection nozzle, thereby heating the injection nozzle with the result that the fuel in the injection nozzle may be excessively heated before being injected.
On the other hand, in the case of the latter internal combustion engine (U.S. Pat. No. 4,876,999), since the fuel from the injection nozzle is injected toward the wall of the air passage communicating with the combustion chamber so as to be impinged thereon and mixed with air, a phenomenon of spitting of fuel toward the air-cleaner, which is disposed for feeding air to the engine, may tend to occur. In such case, unburnt air-fuel mixture may be released through the air-cleaner to the ambient atmosphere, thus producing an environmental problem. Furthermore, since the fuel from the injection nozzle is injected toward a heat conductive wall constituting the cylinder wall, it is difficult to control the air/fuel ratio of the engine, such as the aforementioned lean burn combustion.